Shenzhen Hopetime Industry Co.,Limited
Shenzhen Hopetime Industry Co.,Limited

The PCB Assembly Process-Overview

The PCB Assembly Process-Overview

Taking a PCB from layout to populated board requires 2 stages
1.To manufacture the actual bare PCB 
2.To populate it with components.

BOM and Centroid File: Uploading and Validating Component Data
For the assembly (population) of the PCB we need 2 additional files, these are the BOM (Bill of Materials) and the Centroid File (also known as a Pick and Place file) which can be uploaded at the same time bare board data.
The BOM contains the component information such as MPN (Manufacturers Part Number), which MPN relates to which component designators (C1, C2, R1, R2, IC1 etc), component description, package etc.
The Centroid File defines the position of each component designator on the actual PCB.

Components: Automatic Availability Check and Ordering

Hopetimepcb provide parts sourcing service, We order volume quantities if the parts in your Bill of Materials are common, and none remain in our inventory. If we generally keep your chosen parts in stock, you will pay little or nothing for these components. For uncommon parts listed in your BOM, we usually order 5% or 5 spare parts (depending on size and cost) to accomplish your order without missing parts or shortages due to manufacturing loss.We cover the cost of production loss on our end, so you never have to worry about paying extra. If we encounter any issues with BOM components,such as a large minimum order for a certain part, we will communicate with you to find the best solution.More details click at here:
Components sourcing for PCB Assembly

Front-end Process: Selecting and Checking Components
During the front-end process we check for components that may have special mounting requirements or need a unique reflow profile.
We compare the components package to the footprint on the PCB checking also the orientation and the polarity of the parts.
During the front-end process the programs and files are also prepared for the various assembly machines.
Once all ordered components arrive in-house, they are checked (markings, quantity etc.) and are released to production.
The first production step is Kitting, where we place the components in the feeders and assign the feeder to a basket. These can then be loaded into the magazine of the pick & place systems.

Solder Paste: Direct jetting or SMD Stencil Printing
Once this is complete, we need to apply the solder paste, this can be done either using a stainless-steel stencil or by jetting.
Jetting is like an inkjet printer just more sophisticated.
It can print 300 Solder Paste dots per second and uses optical registration. This is a good solution for prototypes as no stencils are required and provides flexibility in production.
When working with stencils these need to be manufactured and for this, we use a laser cutter for accuracy and speed. We use either 70, 100 or 127 micron thick stencils to achieve the best printing quality.

Solder Paste Printing: Printing the Exact Amount
The actual amount of Solder Paste to be printed on each pad is defined during the front-end process and using this information we can make the stencil or program the jetter.
After the printing process comes the Solder Paste Inspection (SPI). The machine makes a 3D picture to correctly measure the amount of the solder paste on the pad. We start the assembly process only on perfectly pasted panels.

SMD Assembly: Pick & Place of components
We use pick & place machines to mount SMD components on the boards.
Most of the components arrive in tape but the machine also can handle tube or tray packages.
There are 2 types of pick & placement heads. The High-speed head can pick up and place up to 8 smaller and lighter parts at a time. The Single head is designed for bigger and heavier components.
The machine camera system checks every component and compares its dimensions to those in our database. It also has an electrical verification system that measures the value of transistors, resistors, capacitors, diodes and the orientation of polarised components.

Reflow Soldering: The Correct Solder Profile for Optimal Solder Joints
After the components are placed the boards enter a reflow oven for the soldering process which has 4 stages: Preheat, Soak, Reflow and Cooling.
Basically, the oven heats up the panel, activates the flux and melts the paste to create molten solder, it then flows attaching the component pad to the copper pad of the board. It then cools and solidifies making a solid mechanical and electrical connection between the component pad and the board pad.

Optical Inspection: Control and Comparison with the Reference Images
It is used to check the boards before and after reflow, final inspection and component placement etc., the images are then compared to those in our Visualizer.
We use a X-ray machine to check the hidden solder connection as it is very good for process control and failure analysis.
It is an excellent and quick solution for checking BGA and LGA components and connectors when the solder joints are covered. The machine is also capable of laminography, producing images that are easy to interpret though they do take a few hours to produce.

THT Components: Selective Soldering
Through hole components can be either soldered by hand or using our selective soldering machine. And although manual soldering may seem outdated it is still a useful solution for some components.
In most of the cases we use a selective soldering machine as the process is more reliable. It consists of 3 main steps, the application of liquid flux, preheating of the board, and soldering with a specific solder nozzle.
Last, but not least comes the cleaning, final inspection and the shipping of the assembled board. During final inspection we compare the assembled board to the Visualizer to check the details of order.

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